It is essential to properly expose a digital image to obtain a good quality rendition of the original scene on an output device such as a monitor or a printer. The “exposure” of a digital image refers to the quantity of light allowed to act on the image capture sensor; exposure is a product of the intensity (controlled by the aperture and intensity of the illuminant) and the duration (controlled by the shutter speed) of light striking the sensor. Large exposure values will result in brighter images and vice versa. Relying on the original exposure set by the input device (e.g., a digital camera) usually does not yield the best quality for several reasons. For example, a wide variety of picture-taking conditions and scene compositions may make the original exposure quite variable and differ from the preferred exposure. Furthermore, input devices typically have limited dynamic range and therefore err on the side of under-exposing an image to avoid losing information in an image due to clipping. Although underexposed images may appear darker than desired, they tend to retain more information than overexposed images and therefore are amenable to post-acquisition exposure correction to make them more suitable for printing or displaying on an output device.
It is desirable that output devices be equipped to produce properly-exposed renderings from images acquired using a variety of (possibly unknown) image acquisition devices. For example, a desktop digital photo printer or a photo-vending kiosk may be capable of receiving digital images acquired using any of a wide variety of digital cameras, scanners, or other input devices under a wide variety of conditions. It is desirable that such a printer or kiosk be capable of correcting the exposure of any images it receives so that such images may be printed with optimal exposures.
What is needed, therefore, are improved techniques for correcting the exposure of digital images